Various sealing and printing techniques to provide authentication and to avoid unauthorised replication of products and documents are known from the prior art. However, an increasing economic damage results from forgery due to insufficient security.
For authenticating documents and things U.S. Pat. No. 5,145,212 teaches the use of non-continuous reflective holograms or diffraction gratings. Such a hologram or diffraction grating is firmly attached to a surface that contains visual information desired to be protected from alteration. The reflective discontinuous hologram is formed in a pattern that both permits viewing the protected information through it and the viewing of an authenticating image or other light pattern reconstructed from it in reflection. In another specific authentication application of this U.S. Patent a non-transparent structure of two side-by-side non-continuous holograms or diffraction patterns, each reconstructing a separate image or other light pattern, increases the difficulty of counterfeiting the structure.
PCT application WO87/07034 describes holograms, including diffraction gratings, that reconstruct an image which changes as the hologram is tilted with respect to the viewer and in a manner that images reconstructed from copies made of the hologram in monochromatic light do not have that motion.
In UK Patent Application GB 2 093 404 sheet material items which are subject to counterfeiting have an integral or bonded authenticating device which comprises a substrate having a reflective diffractive structure formed as a relief pattern on a viewable surface thereon and a transparent material covering the structure. Specified grating parameters of the diffractive structure result in peculiar, but easily discernable, optical colour properties that cannot be copied by colour copying machines.
U.S. Pat. No. 4,661,983 describes a random-pattern of microscopic lines or cracks having widths in the order of micrometers that inherently forms in a di-electric coating layer of an authenticating device incorporated in a secure document. It permits identification of a genuine individual document by comparing read-out line-position information derived by microscopic inspection with read-out digital codes of line-information obtained earlier at the time of fabrication of the document.
U.S. Pat. No. 5,856,070 shows an authentication label containing a light diffracting structure. Unique parameters are randomly defined in the light diffracting structure by anisotropic process steps not under full control of the producer during the manufacturing of the diffracting structure to prevent copying or creating an exact replica thereof. The resultant uniquely coloured authenticating pattern can be verified by simple observation with the naked eye.
U.S. Pat. No. 4,218,674 shows an authentication method and system that uses an object being of base material having random imperfections. The random imperfections are converted into pulses along a predetermined measuring track over the surface of the object of base material. WO01/57831 shows a similar method that uses random gas enclosures in an authentication object.